(i) Technical Field
The present invention relates to a laminated body containing a particle dispersion layer, a method for producing the same, and a light modulation device.
(ii) Related Art
Various rewritable marking techniques excellent in convenience have been studied, and a light modulation device using a cholesteric liquid crystal as one type thereof receives attention in recent years since it has such characteristics as memory effect for retaining display without power source, bright display owing to absence of polarizing plate, and color display without color filter.
A planar phase shown by a cholesteric liquid crystal (chiral nematic liquid crystal) produces selective reflection phenomenon, in which light incident in parallel to the helical axis is separated into right-handed polarized light and left-handed polarized light, a circularly polarized light component agreeing with the torsional axis of helix is subjected to Bragg reflection, and the remaining light is transmitted. The center wavelength λ and the reflection wavelength width Δλ are expressed by λ=n·p and Δλ=Δn·p, respectively, wherein p represents the helical pitch, n represents the average refractive index in the plane perpendicular to the helical axis, and Δn represents the birefringence, and reflected light from a cholesteric liquid crystal layer in a planar phase shows bright color depending on the helical pitch.
A cholesteric liquid crystal having a positive dielectric anisotropy shows three states, i.e., a planar phase shown in FIG. 15A, in which a helical axis is perpendicular to the cell surface, whereby the incident light is subjected to the aforementioned selective reflection phenomenon, a focal conic phase shown in FIG. 15B, in which the helical axis is substantially in parallel to the cell surface, whereby the incident light is transmitted with slight amount of forward scattered light, and a homeotropic phase shown in FIG. 15C, in which the helical structure is raveled out to make the liquid crystal director directed to the direction of the electric field, whereby the incident light is transmitted substantially completely.
Among the three phases, the planar phase and the focal conic phase can be present bistably under no electric field. Accordingly, the phase state of a cholesteric liquid crystal cannot be determined unconditionally with respect to the intensity of the electric field applied to the liquid crystal layer, and in the case where the initial state is a planar phase, it is changed from a planar phase to a focal conic phase and a homeotropic phase in this order upon increase in electric field intensity, and in the case where the initial state is a focal conic phase, it is changed from a focal conic phase to a homeotropic phase upon increase in electric field intensity.
In the case where the intensity of the electric field applied to the liquid crystal layer is quickly removed, a planar phase and a focal conic phase maintain their states, respectively, and a homeotropic phase is changed to a planar phase.
Accordingly, a cholesteric liquid crystal layer immediately after applying with a pulse signal shows a switching behavior shown in FIG. 16, in which a selective reflection state, in which a homeotropic phase is changed to a planar phase, is produced in the case where the voltage of the pulse signal applied is Vfh or more, a transmission state with a focal conic phase is produced in the case where the voltage is between Vpf and Vfh, and the state before application of the pulse signal, i.e., a selective reflection state with a planar phase or a transmission state with a focal conic phase, is maintained in the case where the voltage is Vpf or less.
In FIG. 16, the ordinate shows the normalized light reflectivity, which is normalized with the maximum reflectivity as 100 and the minimum reflectivity as 0. Because transition areas are present among the planar phase, the focal conic phase and the homeotropic phase, the case where the normalized reflectivity is 50 or more is designated as the selective reflection state, and the case where the normalized reflectivity is less than 50 is designated as the transmission state, with Vpf as the threshold voltage of phase change between the planar phase and the focal conic phase and Vfh as the threshold voltage of phase change between the focal conic phase and the homeotropic phase.
The cholesteric liquid crystal device may have a structure, in which a liquid crystal is sealed as a continuous phase between a pair of display substrates, and may also have a PDLC (polymer dispersed liquid crystal) structure, in which a cholesteric liquid crystal is dispersed as droplets in a polymer binder, and a PDMLC (polymer dispersed microencapsulated liquid crystal) structure, in which a microencapsulated cholesteric liquid crystal is dispersed in a polymer binder.
The use of the PDLC structure and the PDMLC structure reduces flowability of the liquid crystal to suppress disorder in images due to bending and pressure, whereby a flexible medium is realized. Furthermore, color display is realized by directly laminating plural cholesteric liquid crystal layers, and a display device addressing images with light signal can be produced by laminating a photoconductor layer to the liquid crystal layer. Moreover, the display layer can be formed by using a thick film printing technique, whereby the production method can be simplified to save the production cost.